In cases of chronic kidney failure, various methods of cleansing or treating blood by means of apparatus are used to remove substances needing to be excreted and to withdraw fluid. In hemodialysis (HD), the predominant mechanism is the diffusive movement of substances, whereas in hemofiltration (HF) a convective movement of substances takes place through the semi-permeable membrane of the dialyzer or filter. Hemodiafiltration (HDF) is a combination of these two methods. When mention is made of a dialyzer in what follows, what a dialyzer is also understood to mean is a filter.
Because of the large amounts of material exchanged, there is in the above-mentioned methods a need for the fresh dialysis fluid fed to the dialyzer and the used dialysis fluid taken away from the dialyzer to be exactly balanced, with due allowance made for the quantity of fluid which is withdrawn from the patient through the membrane of the dialyzer, over the treatment time as a whole. The prior art includes gravimetric and volumetric balancing systems. In controlled dehydration, some or all of the amount of fluid which is withdrawn from the patient by ultrafiltration can be fed back to the patient in the form of pyrogen-free dialysate or substituate. The infeed of pyrogen-free dialysate or substituate to the extra-corporeal blood circuit at a point upstream of the dialyzer is referred to as predilution and the infeed of dialysate or substituate at a point downstream of the dialyzer is referred to as postdilution. The two methods may also be combined with one another.
DE A 28 38 414 describes a dialysis apparatus having a balancing system which has two balancing chambers, halves of which are filled alternately with fresh dialysis fluid while used dialysis fluid is expelled from whichever are the other halves of the chambers at the time. The part of the fluid circuit which is included between the balancing system and the dialyzer therefore acts as a closed system whose volume is constant. Fluid can be withdrawn from the closed system by means of an ultrafiltration pump. If fluid is withdrawn, a pressure gradient is set up at the semi-permeable membrane of the dialyzer and this gradient results in the same amount of fluid being withdrawn from the patient as is withdrawn from the closed system by the ultrafiltration pump.
For dialysis treatment, it is very important for the amount of fluid withdrawn from the patient by ultrafiltration to be able to be determined exactly. Very stringent requirements are therefore set for the accuracy of the balancing process. However, this presupposes that it is possible to achieve an exact balance of fresh and used dialysis fluid with the balancing chambers of the balancing system. Even slight errors in the balancing in the successive phases of operation of the balancing system may build up over the time taken by the treatment as a whole to misbalances which are no longer acceptable.
Volumetric balancing systems are known in which fresh and used dialysis fluid are fed respectively into and out of the balancing chambers by pumps. What are also known however are balancing systems in which balancing chambers divided into two halves by a membrane have a working fluid applied to them, to feed fresh or used dialysis fluid as the case may be.
DE A 198 30 928 describes a dialysis apparatus having a volumetric balancing system whose balancing chambers take the form of disposable items intended for once-only use. The balancing system comprises a total of four balancing chambers which are each divided into two halves by a membrane. For fresh or used dialysis fluid which is situated in one of the two halves of the chamber to be fed, a working fluid is applied to the other half of the chamber. The four balancing chambers form a first pair of balancing chambers which are intended to feed fresh dialysis fluid, and a second pair of balancing chambers which are intended to feed used dialysis fluid. The fresh dialysis fluid is fed by the two balancing chambers forming one pair of balancing chambers from a source of dialysis fluid to the dialysis-fluid chamber of the dialyzer. The used dialysis fluid from the dialysis-fluid chamber of the dialyzer is fed by the two balancing chambers forming the other pair of balancing chambers to a discharge. When this takes place, the halves of alternate balancing chambers are filled with fresh and used dialysis fluid in the respective cases, thus causing there to be a continuous flow of dialysis fluid through the dialysis-fluid chamber of the dialyzer.
To enable the respective balancing chambers to be filled alternately with fresh and used dialysis fluid, the relevant halves of the balancing chambers are connected, in the respective cases, into the infeed line for fresh dialysis fluid which runs from the source of dialysis fluid to the dialysis-fluid chamber of the dialyzer and into the takeaway line for used dialysis fluid which leads away from the dialysis-fluid chamber of the dialyzer and runs to the discharge. Situated in the respective segments of the infeed and takeaway lines are shut-off members to enable the flow of fluid to be controlled. The flow management and control system as a whole is neither intended nor suitable to allow the balancing chambers forming one pair of balancing chambers, which are intended to feed fresh dialysis fluid, to be used to feed used dialysis fluid and vice versa.
It is a disadvantage that, for exact balancing of fresh and used dialysis fluid, it is a prerequisite for the volumes of the individual halves of the balancing chambers to be exactly the same. This however makes great demands on the production tolerances, which can only be maintained with a great deal of technical cost and complication, particular in the case of an arrangement of balancing chambers which is designed to be a disposable item.
Also known, from US 2005/0000868 A1, is a balancing system which has four balancing chambers which are used to balance against one another the substituate or replacement fluid which is being fed to the patient and the filtrate or waste fluid which is withdrawn via the membrane of the dialyzer. In the case of this balancing system too, two chambers in each case form a pair of chambers which receive either substituate or filtrate. Pumps in the infeed and takeaway lines serve to fill and empty the chambers. For this reason the chambers themselves do not need to be divided into two halves by a membrane.